This invention relates to an elastic surface wave device, in particular to an elastic surface wave device comprising a multi-strip coupler or hone type acoustic beam width compresser combined with an interdigital transducer.
In conventional art, an elastic surface wave device is produced by depositing a thin metal film on a surface of piezo-electric substrate in which an interdigital transducer is designed. In the interdigital transducer, the electrodes are insulated from each other by being arranged in a comb type configuration as shown in FIG. 1, thereby forcing the electrical signal to proceed along the electric substrate surface, on which a thin metal film is deposited, when an electrical signal is applied to the input portion of the interdigital transducer. The signal velocity proceeding on the surface of the piezo-electric material moves slower by ##EQU2## in comparison to that of the electro magnetic wave signal. Thus, with the above piezo electric device comprising a small configuration, the velocity of an F.M. signal can be delayed because the center frequency of such elastic surface wave device is determined by the width of the comb-type electrode.
Thus if the elastic surface wave device is used, it is not necessary to tune the center frequency as in the conventional R-L-C circuit.
Further, the size of the pizo-electric device is very small i.e. comparable to that of an intergrated circuit board and has a planar configuration. This highly reliable piezo-electric device is very easy and simple to produce by using the photolithography method.
Also, the center frequency and band pass width of the elastic surface wave is determined by the width and number of the comb type electrodes, therefore the elastic surface wave device can be operated at high frequency signals (10 MHz.about.500 MHz) or wide band signals (10 MHz.about.300 MHz).
On the other hand there are well-known techniques for discriminating the frequency modulating signal (F.M. Signal).
For example there are tuned F.M.discriminators provided with a differentiator and an amplitude detector, emitter coupled quadrature F.M. discriminators comprising a low-pass filter, and descriminators comprised of a voltage control oscillator, a phase comparator, and a low pass filter. But the subject F.M. discriminator has a complicated circuit configuration comprising a resistor, coil, capacitor, transistor and diode, and such an F.M. discriminator must be tuned to it's peripheral circuit, and thus is bulky and consumes a lot of power. Before, attempts have been made to produce an improved discriminator by using an integrated circuit, however, such a discriminator could not be used for discriminating frequencies above 30 MHz.
For overcoming the foregoing disadvantages an elastic surface wave device was developed for discriminating FM signals by using a piezo-electric element.
The device uses a non-linear silicon space charge, but has a problem in that it must use two silicon semiconductors formed in a step type dual configuration to retain a predetermined air gap (as disclosed by W. C. Wang in 1978).
Thus, there has always been a need to develop an elastic surface wave device which is simple in configuration, easy to produce, and capable of discriminating high frequencies and wide band signals.
Therefore to solve the aforementioned disadvantages and problems this invention comprises an elastic surface wave device which generates a frequency modulated signal by using a non-linear piezo-electric substrate. With this invention, tuning is unnecessary and its configuration is compact, consisting of a single chip. Further the device consumes little power and can be operated at high frequencies, for example 10 MHz.about.500 MHz and can deal with wide band signals, for example 10 MHz.about.300 MHz.
Thus, when the above device is installed and used in a conventional F.M. receiver (88 MHz.about.108 MHz), the operation transducing the intermediate frequency (I.F.) is not necessary.
Also such a device can be installed as an F.M. discriminator in the DPSK demodulator.